package snippet;

public class Snippet {
	public static double[] PolyFit(double x[], double y[], int n, double a[],
			int m) {
		int i, j, k;
		double z, p, c, g, q = 0, d1, d2;
		double[] s = new double[20];
		double[] t = new double[20];
		double[] b = new double[20];
		double[] dt = new double[3];
		for (i = 0; i <= m - 1; i++) {
			a[i] = 0.0;
		}
		if (m > n) {
			m = n;
		}
		if (m > 20) {
			m = 20;
		}
		z = 0.0;
		for (i = 0; i <= n - 1; i++) {
			z = z + x[i] / (1.0 * n);
		}
		b[0] = 1.0;
		d1 = 1.0 * n;
		p = 0.0;
		c = 0.0;
		for (i = 0; i <= n - 1; i++) {
			p = p + (x[i] - z);
			c = c + y[i];
		}
		c = c / d1;
		p = p / d1;
		a[0] = c * b[0];
		if (m > 1) {
			t[1] = 1.0;
			t[0] = -p;
			d2 = 0.0;
			c = 0.0;
			g = 0.0;
			for (i = 0; i <= n - 1; i++) {
				q = x[i] - z - p;
				d2 = d2 + q * q;
				c = c + y[i] * q;
				g = g + (x[i] - z) * q * q;
			}
			c = c / d2;
			p = g / d2;
			q = d2 / d1;
			d1 = d2;
			a[1] = c * t[1];
			a[0] = c * t[0] + a[0];
		}
		for (j = 2; j <= m - 1; j++) {
			s[j] = t[j - 1];
			s[j - 1] = -p * t[j - 1] + t[j - 2];
			if (j >= 3)
				for (k = j - 2; k >= 1; k--) {
					s[k] = -p * t[k] + t[k - 1] - q * b[k];
				}
			s[0] = -p * t[0] - q * b[0];
			d2 = 0.0;
			c = 0.0;
			g = 0.0;
			for (i = 0; i <= n - 1; i++) {
				q = s[j];
				for (k = j - 1; k >= 0; k--) {
					q = q * (x[i] - z) + s[k];
				}
				d2 = d2 + q * q;
				c = c + y[i] * q;
				g = g + (x[i] - z) * q * q;
			}
			c = c / d2;
			p = g / d2;
			q = d2 / d1;
			d1 = d2;
			a[j] = c * s[j];
			t[j] = s[j];
			for (k = j - 1; k >= 0; k--) {
				a[k] = c * s[k] + a[k];
				b[k] = t[k];
				t[k] = s[k];
			}
		}
		dt[0] = 0.0;
		dt[1] = 0.0;
		dt[2] = 0.0;
		for (i = 0; i <= n - 1; i++) {
			q = a[m - 1];
			for (k = m - 2; k >= 0; k--) {
				q = a[k] + q * (x[i] - z);
			}
			p = q - y[i];
			if (Math.abs(p) > dt[2]) {
				dt[2] = Math.abs(p);
			}
			dt[0] = dt[0] + p * p;
			dt[1] = dt[1] + Math.abs(p);
		}
		return a;
	}

	/**
	 * <p>
	 * 对X轴数据节点球平均值
	 * </p>
	 * 
	 * @param x
	 *            存储X轴节点的数组
	 *            <p>
	 *            Date:2007-12-25 20:21 PM
	 *            </p>
	 * @author qingbao-gao
	 * @return 平均值
	 */
	public static double ave(double[] x) {
		double ave = 0;
		double sum = 0;
		if (x != null) {
			for (int i = 0; i < x.length; i++) {
				sum += x[i];
			}
			System.out.println("sum-->" + sum);
			ave = sum / x.length;
			System.out.println("ave" + ave + "x.length" + x.length);
		}
		return ave;
	}

	/**
	 * <p>
	 * 由X值获得Y值
	 * </p>
	 * 
	 * @param x
	 *            当前X轴输入值,即为预测的月份
	 * @param xx
	 *            当前X轴输入值的前X数据点
	 * @param a
	 *            存储多项式系数的数组
	 * @param m
	 *            存储多项式的最高次数的数组
	 *            <p>
	 *            Date:2007-12-25 PM 20:07
	 *            </p>
	 *            <P>
	 *            Author:qingbao-gao
	 *            </P>
	 * @return 对应X轴节点值的Y轴值
	 */
	public static double getY(double x, double[] xx, double[] a, int m) {
		double y = 0;
		double ave = ave(xx);

		double l = 0;
		for (int i = 0; i < m; i++) {
			l = a[0];
			if (i > 0) {
				y += a[i] * Math.pow((x - ave), i);
				System.out.println(i + "--|-->" + y + "--a[i]--" + a[i]);
			}
			System.out.println("a[0]|" + a[0]);
		}
		System.out.println("l--|" + (l));
		return (y + l);
	}

	// --------------------------------------------测试代码
	public static void main(String[] args) {

		double[] x = { 200401, 200402, 200403, 200404, 200405, 200406, 200407,
				200408, 200409, 2004010, 2004011, 2004012, 200501, 200502,
				200503, 200504 };
		double[] y = { 51, 51, 53, 53, 54, 55, 57, 60, 63, 64, 66, 66, 69, 71,
				72, 75 };
		double[] a = new double[20];
		double[] aa = PolyFit(x, y, 16, a, 3);
		double yy = 0;
		System.out.println("拟合-->" + getY(200505, x, aa, 3));

	}
}
